In a magnetic disk or tape data storage device, data is commonly stored on a magnetic medium by saturation recording in which each portion of the medium is magnetized to the point of saturation in one of two directions. The data to be stored is typically encoded to satisfy certain constraints and the encoded data is used to modulate the direction of magnetization. In a coded representation known as NRZI, each "one" bit of the encoded data causes a transition in the direction of magnetization, while each "zero" bit of the encoded data causes the magnetization direction to remain unchanged. A clock signal is used to write a sequence of encoded NRZI bits as a recording head moves along a track on the medium such that one bit is written at each clock tick.
When a read head is passed over the recorded data track, a voltage pulse is produced at each transition in magnetization. Successive voltage pulses have opposite polarity since successive magnetic transitions are in opposite directions. The NRZI data sequence that was written may be reconstructed from the resulting voltage waveform by associating a "one" bit with every clock tick at which a pulse occurs and a "zero" bit with every clock tick at which no pulse occurs. The original user data may then be decoded from the NRZI data.
Similar waveforms composed of pulses sometimes occur in other applications, including data storage on optical media and data communications via modem, network, radio, or fiber-optic link.
To recover the written or transmitted data sequence, the receiver must be able to detect and time the pulses read. This requires that the amplitude of the pulses be controlled so that the pulse sent to the detection circuit and the timing recovery circuit has a consistent level.
Inter-symbol interference (ISI) occurs when pulses are placed so close together in a waveform that they overlap significantly. The position of a peak in a waveform can be shifted by ISI from other nearby pulses. Thus the recording density of a peak detection channel is limited by the fact that ISI must not be permitted to move a peak from one window to another. ISI also causes amplitude variations that must be adjusted for by a gain control circuit.